The power output of lasers is typically limited by catastrophic damage to the components due to the high intracavity intensities. Use of a master-oscillator/power-amplifier (MOPA) configuration allows the power to be increased, but for amplifiers of finite cross sectional area, the output power can still limited by damage at the exit face. Further increases in the power of a MOPA system can be obtained by combining multiple amplifiers in parallel. For incoherent combination, there is no fixed phase relationship between the electromagnetic field emitted by the various amplifiers. For coherent combination, there is. If, in addition, the emitted wavefronts are planar and in-phase, the divergence of the output will be smaller for coherent combination than it would be for incoherent combination. This comparison assumes the same wavelength and total aperture size in both cases. A smaller divergence for coherent combination leads to a smaller spot in the far-field, or in the focal plane of a lens. The same power in a smaller spot means a higher intensity on the sample or target.
For both coherent and incoherent combination, spatial variations in the intensity of the output beams will increase the divergence. Even if the intensities are spatially uniform, if there are gaps between the multiple apertures, e.g., when circular apertures are arranged in a hexagonal pattern, the divergence will increase. For this reason, it is sometimes desirable to spatially overlap the output beams, thereby increasing the fill factor and decreasing the divergence.
Electronic control of the relative phases of the multiple outputs allows for a degree of beam steering without the need for a moving mirror. Such a system is called a phased array. Beam steering with a phased array can be used to engage a moving target, for example. Usually it is done with collimated beams and no additional optics.